The present invention relates generally to telecommunication techniques. More particularly, the invention provides a method and apparatus for fast mapping of Code Excited Linear Prediction (CELP) model parameters. Merely by way of example, the invention has been applied to voice transcoding from one CELP coder/decoder (codec) to another CELP codec, but it would be recognized that the invention has a much broader range of applicability.
Code Excited Linear Prediction (CELP) speech coding techniques are widely used for speech codecs. Such codecs model voice signals as a source filter model. The source/excitation signal is generated via adaptive and fixed codebooks, and the filter is modeled by a short-term linear predictive coder (LPC). The encoded speech is then represented by a set of parameters which specify the filter coefficients and the type of excitation. Parameters of a CELP codec include the line spectral pair (LSP) parameters, adaptive codebook parameters, and fixed codebook parameters.
Industry standards codecs using CELP techniques include Global System for Mobile (GSM) Communications Enhanced Full Rate (EFR) codec, Adaptive Multi-Rate Narrowband (AMR-NB) codec, Adaptive Multi-Rate Wideband (AMR-WB), G.723.1, G.729, Enhanced Variable Rate Codec (EVRC), Selectable Mode Vocoder (SMV), QCELP, and MPEG-4. A transcoding process can convert CELP parameters from one voice compression format to another voice compression format. Some transcoding techniques fully decode the compressed signal back to a Pulse-Code Modulation (PCM) representation and then re-encode the signal. These techniques usually use a large amount of processing and incur significant delays. Other transcoding techniques convert CELP parameters from one compression format to the other while remaining in the parameter space. These techniques usually use complex computation that is prone to overflow errors.
Hence it is desirable to improve CELP transcoding techniques.